U.S. patent number 4,596,903 [Application Number 06/604,493] was granted by the patent office on 1986-06-24 for pickup device for picking up vibration transmitted through bones.
This patent grant is currently assigned to Pilot Man-Nen-Hitsu Kabushiki Kaisha. Invention is credited to Naomi Yoshizawa.
United States Patent |
4,596,903 |
Yoshizawa |
June 24, 1986 |
Pickup device for picking up vibration transmitted through
bones
Abstract
A pickup device for picking up vibration transmitted through
bones or solid members in human, in which a piezoelectric element
is mounted like a cantilever within a substantially rigid
liquid-tight case filled with a viscous liquid in such a way that
the free end of the piezoelectric element can vibrate with respect
to the liquid-tight case. The viscosity of the viscous liquid
ranges from 30 to 10,000 cs so that the sharpness of resonance of
the piezoelectric element can be suitably suppressed.
Inventors: |
Yoshizawa; Naomi (Hatano,
JP) |
Assignee: |
Pilot Man-Nen-Hitsu Kabushiki
Kaisha (Tokyo, JP)
|
Family
ID: |
13667381 |
Appl.
No.: |
06/604,493 |
Filed: |
April 27, 1984 |
Foreign Application Priority Data
|
|
|
|
|
May 4, 1983 [JP] |
|
|
58-78637 |
|
Current U.S.
Class: |
381/151; 367/166;
367/171; 381/173 |
Current CPC
Class: |
G01H
11/08 (20130101); H04R 17/02 (20130101); H04R
1/46 (20130101) |
Current International
Class: |
G01H
11/08 (20060101); G01H 11/00 (20060101); H04R
17/02 (20060101); H04R 1/46 (20060101); H04R
1/00 (20060101); H04R 017/02 () |
Field of
Search: |
;179/121C,17BC,11A
;367/157,166,171 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Rubinson; Gene Z.
Assistant Examiner: Byrd; Danita R.
Attorney, Agent or Firm: Wenderoth, Lind & Ponack
Claims
What is claimed is:
1. A pickup device for detecting auditory vibrations transmitted
through bones or other rigid members in a human body,
comprising:
a substantially rigid liquid-tight case having a cavity therein, a
cantilever-like piezo-electric element having a first end fixed to
said case so as to vibrate therewith when said case vibrates in
response to auditory vibrations, and a free second end extending
into said cavity; and
a viscous liquid having a viscosity in the range 30 cs to 10,000
cs, filling said cavity so as to dampen resonant vibrations of said
free end of said piezo-electric element;
said piezo-electric element having a resonant frequency greater
than 1.5 KHz and exhibiting a Q in said device less than 3.
2. A pickup device as in claim 1, wherein said viscous liquid is a
liquid selected from the group of viscous liquids consisting of
paraffin chloride, high-molecular plasticizer, glycol, mineral oil,
motor oil, organic solvent, prepolymer and epoxy resin.
3. A pickup device as in claim 1, wherein said case has a small
hole for filling said cavity with said viscous liquid and a plug
closing said small hole.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a pickup device which is best
adapted to pickup high quality audio signals transmitted through
bones with a minimum level of noise and without causing
howling.
Pickup devices for picking up vibration transmitted through bones
of the type in which a piezo-electric element which is supported
like a cantilever picks up audio signals transmitted through bones
have been well known in the art. For instance, Japanese Patent
Publication No. 39763-1978 discloses a pickup device of the type
described above. When one inserts such a pickup device into an ear
and speaks, the pickup device picks up noise when the surrounding
noise level is in excess of 90 dB (A) so that the audio signal
cannot be distinctly distinguished from noise. Furthermore, when
this pickup device is connected to a loudspeaker, a howling noise
is heard very frequently because the audio frequency range includes
resonance frequencies of a few orders so that noise whose frequency
is close to such resonance frequencies is picked up.
In order to solve the above and other problems encountered in the
prior art pickup devices, there has been proposed a method for
using a low-pass filter that transmits frequencies lower than the
resonance frequencies. However, it is extremely difficult to
attenuate only the sensitivity at the resonance frequencies without
attenuating the sensitivity of the audio frequency range because
the sensitivity of a piezo-electric element which is supported like
a cantilever has a high degree of sensitivity to a resonance
frequency.
The inventor has proposed the use of a notch filter in order to
solve the above-described problem. The use of a notch filter is
very effective in decreasing only the peak value of a resonance
frequency, but high volume production cannot be attained because
pickup devices have their own unique resonance frequencies so that
the notch frequency of each notch filter must be matched with the
resonance frequency of each pickup device.
SUMMARY OF THE INVENTION
The inventor made extensive studies and experiments in order to
solve the problem described above and has succeeded in providing a
pickup device for picking up high quality audio signals transmitted
through bones with a minimum level of noise and without causing
howling by causing the vibration of a cantilever-like
piezo-electric element in a viscous fluid of a predetermined
viscosity range so that the sensitivity or response at a resonance
frequency is attenuated to a given value. More particularly, the
present invention provides a pickup device for picking up vibration
or audio signals transmitted through bones of the type in which a
piezo-electric element is supported like a cantilever (to be
referred to as "a cantilever-like piezo-electric element" for
brevity in this specification) in a liquid-tightly enclosed case
filled with a viscous liquid with a viscosity ranging from 30 to
10000 cs so that the peak resonance level Q of the piezo-electric
element in the viscous liquid is lower than 3 when a resonance
frequency is in excess of 1.5 KHz.
In a pickup device in accordance with the present invention, a
cantilever-like piezo-electric element is enclosed in a
liquid-tight case and a supporting member which supports one end of
the cantilever-like piezo-electric element is securely attached to
the liquid-tight case.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a longitudinal sectional view of a first embodiment of a
pickup device in accordance with the present invention;
FIG. 2 is a longitudinal sectional view of a second embodiment of
the present invention;
FIG. 3 is a sectional view taken along the line III--III of FIG.
2;
FIG. 4 is a sectional view taken along the line IV--IV of FIG.
2;
FIG. 5 shows the frequency characteristic curves of pickup devices
when no viscous liquid is used and when viscous liquids are
used;
FIG. 6 shows the frequency characteristic curves of pickup devices
when no viscous liquid is used and the frequency characteristic
curves when Q is 3; and
FIG. 7 shows the relationship among the resonance frequency,
dynamic viscosity and Q factors.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, the resonance frequency of a
cantilever-like piezo-electric element must be higher than 1.5 KHz.
If the resonance frequency is lower than 1.5 KHz, articulation or
intelligibility becomes less than 60% and the sentence
intelligibility drops less than 88% so that the function of a
pickup device for picking up vibration transmitted through bones is
lost. Piezo-electric elements used in the present invention may be
made of barium titanate or lead zirconate and it is preferable to
use a bimorph cell. One end of a piezo-electric element can be
supported like a cantilever or a piezo-electric element can be
bonded to one surface of a cantilever made of a metal, plastic,
ceramic or the like. As long as a piezo-electric element is
supported like a cantilever, the shape of the piezo-electric
element is not limited, but it is preferable to use a
cantilever-like piezo-electric element in the form of a plate, a
rod or wire. The term "piezo-electric element" used in this
specification also refers to a piezo-electric element of the type
in which a piezo-electric element is bonded to one surface of a
metal, plastic or ceramic cantilever.
In general, it is well known that the longer and thinner a
cantilever is, the lower the resonance frequency becomes. It
therefore follows that a piezo-electric element with a
predetermined resonance frequency can be obtained by changing the
length, thickness and shape. For instance, in the case of a bimorph
cell of the type in which piezo-electric elements are bonded to
both surfaces of a metal plate, it is preferable that the length of
the metal plate is less than about 30 mm and the thickness is
greater than about 0.3 mm.
It is preferable to use a hard material to fabricate a supporting
member which supports one end of a piezo-electric element because
the supporting member made of a hard material can securely support
the piezo-electric element. Furthermore, it is preferable that the
supporting member is made of an electrically insulating material.
For instance, it may be made of plastic, a metal coated with an
insulating material, ceramic or the like. It is further required
that the supporting member is not attacked by a viscous liquid
which is in contact with the supporting member. One end of a
piezo-electric element can be fitted into a recess or hole of the
supporting member.
A liquid-tight case into which is encased a cantilever-like
piezo-electric element supported by a supporting member may be made
of any material such as a metal, plastic, ceramic or the like which
is not attacked by a viscous liquid filling in the case. The
construction of a liquid-tight case may be such that one end of the
case is closed while the other end thereof is open but
liquid-tightly closed with a supporting member which may be press
fitted, bonded or caulked.
In order to facilitate injecting of a viscous liquid into a
liquid-tight case and to prevent the leakage of the viscous liquid
from the liquid-tight case, a small hole is formed at one end of
the liquid-tight case and after the viscous liquid has been
injected into the liquid-tight case, the small hole is
liquid-tightly closed with a ball or a plug. It is preferable that
lead wires extended from the piezo-electric element are coated with
plastic in order to prevent the leakage of a viscous liquid along
them. Moreover, depending upon the intended use, the liquid-tight
case can be covered with a suitable material.
After extensive studies and experiments, the inventor found out the
fact that when a cantilever-like piezo-electric element is immersed
in a viscous liquid, howling is avoided and the noise pickup can be
reduced to a minimum so that the audio signal can be distinctly
distinguished even when the noise level is high. It was also found
out that the higher the viscosity of a liquid, the lower the
sharpness of resonance f.sub.o /.DELTA.f becomes (where f.sub.o is
a resonance frequency and .DELTA.f is a frequency width at which
the sensitivity is lower than the sensitivity at the resonance
frequency f.sub.o by 3 dB and the sharpness of resonance f.sub.o
/.DELTA.f is referred to as "a Q factor" in this specification). As
a result, the performance of a pickup device can be remarkably
improved. For instance, when a cantilever-like piezo-electric
element whose resonance frequency is 4.3 KHz when the element is
not immersed in a viscous liquid, is immersed in various liquids
having a viscosity ranging from 10 to 10000 cs, the Q factor is
dropped from 29 to as low as 0.3 as shown in FIG. 5. In FIG. 5, the
frequency characteristic curve A is that of a pickup device which
does not use a viscous liquid and Q is 29. When liquids or silicon
oils with viscosity of 10, 100, 500, 1000 and 10000 cs,
respectively, are used, the frequency characteristic curves are
represented by curves B, C, D, E and F, respectively. The Q factors
are 8.0, 3.2, 1.2, 0.7 and 0.3, respectively.
After extensive studies and experiments, the inventor succeeded in
providing a pickup device which can distinctly distinguish the
audio signal even when the noise level is as high as 100 dB (A)
when a piezo-electric element is immersed in a viscous liquid with
the viscosity ranging from 30 to 10,000 cs.
The present invention is characterized in that a viscous liquid
with a viscosity ranging from 30 to 10,000 cs is used and that the
Q factor of a piezo-electric element immersed in the viscous liquid
is less than 3.
The Q factor of a cantilever-like piezo-electric element is
dependent upon the length and thickness of the element. According
to the experiments conducted by the inventor, it is found out that
the Q factors of cantilever-like piezo-electric elements with a
resonance frequency higher than 1.5 KHz are greater than 3 as shown
in FIG. 6 (in which the frequency characteristic curves of
piezo-electric elements with a Q factor being 3 are shown in dotted
lines). Even when a piezo-electric element is securely enclosed
within a liquid-tight case and a viscous liquid fills the case so
that the Q factor is decreased, noise whose frequency is closer to
a resonance frequency is picked up when the Q factor is higher than
3 and howling occurs and sound quality is not satisfactory. It
therefore follows that in order to attain the objects of the
present invention, the Q factor of a cantilever-like piezo-electric
element must be less than 3.
Various pickup devices are prepared which comprise a
cantilever-like piezo-electric element enclosed in a liquid-tight
case in such a way that a resonance frequency becomes higher than
1.5 KHz. Various kinds of viscous liquids were injected into the
cases so that various Q factors were obtained. FIG. 7 shows the
region in which the Q factor is in excess of 3 and the region in
which the Q factor is less than 3. As is clear from FIG. 7, in
order that a cantilever-like piezo-electric element with a
resonance frequency of 1.5 KHz may have a Q factor less than 3, the
viscosity of a viscous liquid must be higher than 30 cs. It may be
also noticed that the higher the viscosity, the lower the Q factor
becomes. However, if the viscosity of a liquid is in excess of
10,000 cs, it becomes difficult to inject the liquid into a case
and bubbles are produced so that the frequency characteristic
varies. It therefore follows that it is preferable that the
viscosity of a liquid used ranges from 30 to 10000 cs.
Viscous liquids which may be used in the present invention may be
paraffin cloride, high-molecular plasticizers, liquid surface
active agents, glycols, silicon oil, motor oil, other mineral oils,
organic solvents such as hydrocarbons or prepolymers such as
urethane, epoxy resin or the like. It is preferable that the
viscous liquids used in the present invention will not attack a
liquid-tight case and other members in contact with the liquids,
have less odor and will not change their viscosity in response to
the temperature change.
When such viscous liquid fills in a liquid-tight case, a pickup
device for picking up vibration transmitted through bones or
resilient bodies can be provided.
Only the portion of a cantilever-like piezo-electric element which
vibrates may be immersed in a viscous liquid. Alternatively, the
whole cantilever-like piezo-electric element, that is, a
piezo-electric element and its supporting member, can be completely
immersed in a viscous liquid.
The lead wires extended from a piezo-electric element of a pickup
device in accordance with the present invention can be connected to
an amplifier so that the pickup device can be used as a
conventional microphone.
The pickup device in accordance with the present invention may be
inserted into an ear or attached to part of a human body so that it
may be used as a microphone for picking up the audio signal or
voice transmitted through bones. Alternatively, it may be attached
to an accessory or the like worn by a man so that it may be used as
a microphone which indirectly picks up the audio signal or voice
transmitted through bones. Furthermore, the pickup device may be
bonded to a desk or a wall so as to pick up the audio signal or
vibration transmitted therethrough.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The preferred embodiments of the present invention will be now
described with reference to the accompanying drawings, but it is to
be understood that the present invention is not limited
thereto.
FIG. 1 shows a first embodiment of a pickup device in accordance
with the present invention which comprises a substantially rigid
cylindrical brass case 5 consisting of a small-diameter portion 2
having a small hole 1 at one or free end thereof and a
large-diameter portion 4 with an open end 3. One end of a
piezo-electric element 6 is securely supported by a supporting
member 7 and is inserted into the small-diameter portion 2 in such
a way that the supporting member 7 is liquid-tightly seated at the
bottom of the large-diameter portion 4. A viscous liquid 8 is
injected through the small hole 1 into the small-diameter portion 2
and thereafter a ball 9 made of a hard metal such as tungsten
carbide is inserted into the small hole 1. The free end of the
small-diameter portion 2 is caulked so as to hold the ball 9 in
position and to liquid-tightly close the free end. Thus, the case 5
becomes a liquid-tight case.
The open end 3 of the large-diameter portion 4 is closed with a
cover 11 with a cord 10 and lead wires 12 and 13 of the cord 10
through the cover 11 connected to the piezo-electric element 6
through FETs and resistors (not shown). The lead wire 13 is
soldered to the large-diameter portion 4 and is therefore grounded.
In order to ensure the liquid-tightness between the supporting
member 7 and the bottom of the large-diameter portion 4, a sealant
such as silicon, a potting material, an O-ring or a packing may be
used.
In FIGS. 2 through 4 is shown a second embodiment of a pickup
device in accordance with the present invention. A base plate 14 is
formed with two mounting holes 15 and 16 which are spaced apart
from each other by a predetermined distance in the longitudinal
direction. A field-effect transistor (FET) 17 is securely mounted
in the hole 15 and a resistor 18 is securely bonded or otherwise
joined to the undersurface of the base plate 14. A supporting
member 21 is formed with a recess 19 and a projection 20 which are
extended in the same direction (See FIG. 4) and one end 6.sub.1 of
a piezo-electric element 6 is securely fitted into the recess 19.
The piezo-electric element 6 is extended in parallel with the base
plate 14 and spaced apart from the upper surface thereof by a
predetermined distance. The projection 20 is fitted into mounting
hole 16.
The terminals 22, 23 and 24 of the field-effect transistor 17 are
connected to the conduction layers 25, 26 and 27, respectively, of
the undersurface of the base plate 14. The conduction layers 25, 26
and 27 are electrically isolated from each other. The resistor 18
is bridged between the conduction layers 26 and 27. The conduction
layer 25 is connected through a small hole 28 to a conduction layer
29 formed over the upper surface of the base plate 14. In like
manner, the conduction layer 27 is connected through a small hole
30 to a conduction layer 31 formed over the upper surface of the
base plate 14. Lead plates 32 and 33 which are attached to the
upper and lower surfaces, respectively, of the end portion 6.sub.1
of the piezo-electric element 6 are connected to the conduction
layers 31 and 27, respectively.
One shield 36 of a cord 35 carried by a brass cap 34 is soldered to
the inner surface of the cap 34 and is therefore grounded. The
other shield 37 and a lead wire 38 are connected to the conduction
layers 26 and 29, respectively. A filler (not shown) such as epoxy
resin fills the cap 34 so that the cap 34 can be made
liquid-tight.
A substantially rigid brass case 42 is substantially rectangular in
cross section and has flat top and bottom surfaces 39. One end of
the brass case 42 is formed with a small hole 40 while the other
end thereof is opened as indicated by the reference numeral 41. The
base plate 14 is inserted through the open end 41 into the brass
case 42 in such a way that the piezo-electric element 6 can vibrate
at right angles with respect to the flat top and bottom surfaces 39
of the brass case 42. The side edges of the base plate 14 are
forced to press against the inner side wall surfaces of the brass
case 42 and the cap 34 is inserted into the open end 41. Thereafter
the open end 41 is caulked so that the cap 34 can be securely held
in position. Part of the cap 34 is soldered to the open end 41 of
the brass case 42 so that the electrical connection can be
established therebetween. The space between the open end 41 of the
brass case 42 and the cap 34 is filled with an adhesive or the like
so that the cap 34 can liquid-tightly seal the open end 41 of the
brass case 42.
A viscous liquid 8 is injected through the small hole 40 into the
brass case 42 and thereafter a ball 9 made of a hard metal is
fitted into the small hole 40. Next the end with the small hole 40
of the brass case 42 is caulked so that the ball 9 can be held
securely in position and the brass case 42 is liquid-tightly
sealed.
Instead of the ball 9, a plug made of plastic or rubber may be
used.
Next some examples of the present invention will be described.
Examples 1 through 24 are of the type as shown in FIG. 1 comprising
various kinds of piezo-electric elements and viscous liquids.
EXAMPLE 1
One end of a bimorph cell 6 (the product of Fuji Ceramic K.K.)
having the length of 8 mm, the thickness of 0.6 mm and the width of
1.5 mm is securely fixed to the supporting member 7. The inner
diameter of the small-diameter portion 2 is 2 mm and a silicon oil
(the product of Shinetsu Kagaku Kogyo K.K. sold under the trademark
KF-96) is injected into the small-diameter portion 2. Example 1 has
a Q factor of 2.5.
In Examples 2 through 24, the piezo-electric elements are the
products of Fuji Ceramic K.K.; and the silicon oil is the product
of Shinetsu Kagaku Kogyo K.K. sold under the trademark KF-96 while
the surface active agent is the product of Kao Sekken K.K. sold
under the trademark Emulgen 810.
______________________________________ Ex- Piezoelectric element
Viscous liquid ample thickness width length viscosity No. (mm) (mm)
(mm) kind (cs) Q ______________________________________ 2 0.6 1.5
12 silicon oil 50 2.2 3 " " " silicon oil 100 1.3 4 " " 9 silicon
oil 100 2.1 5 " " 12 silicon oil 300 0.4 6 " " 9 silicon oil " 0.8
7 " " 8 silicon oil " 1.2 8 " " 6.6 silicon oil " 1.7 9 " " 12
surface active 550 0.3 agent 10 " " 9 surface active " 0.6 agent 11
" " 8 surface active " 0.8 agent 12 " " 6.6 surface active " 1.2
agent 13 " " 12 silicon oil 1000 0.3 14 " " 9 silicon oil " 0.4 15
" " 8 silicon oil " 0.5 16 " " 6.6 silicon oil " 0.7 17 " " 12
paraffin 2600 0.3 chloride 18 " " 9 paraffin " " chloride 19 " " 8
paraffin " " chloride 20 " " 6.6 paraffin " " chloride 21 " " 12
silicon oil 6000 " 22 " " 9 silicon oil " " 23 " " 8 silicon oil "
" 24 " " 6.6 silicon oil "
______________________________________
Examples 25 through 44 are of the type as shown in FIGS. 2-4
comprising various kinds of piezo-electric elements and viscous
liquids.
EXAMPLES 25
A bimorph cell (the product of Fuji Ceramics K.K.) with the
thickness of 0.6 mm, the width of 1.5 mm and the length of 12 mm
has its one end fixed to the supporting member 21 and is inserted
into the brass case 42 with the inside width of 6 mm and the inside
thickness of 3 mm. The silicon oil with the viscosity of 50 cs (the
product of Shinetsu Kagaku K.K. sold under the trademark KF-96) is
injected into the brass case 42. The Q factor is 2.2.
In the following examples, the piezo-electric elements are the
products of Fuji Ceramics K.K. and the silicon oil is the product
of Shinetsu Kagaku Kogyo K.K. sold under the trademark KF-96
series.
______________________________________ Piezoelectric element
Viscous liquid Example thickness width length viscosity No. (mm)
(mm) (mm) kind (cs) Q ______________________________________ 26 0.6
1.5 12 silicon oil 100 1.3 27 " " 9 " " 2.1 28 " " 8 " " 2.5 29 " "
12 " 300 0.4 30 " " 9 " " 0.8 31 " " 8 " " 1.2 32 " " 6.6 " " 1.7
33 " " 12 " 500 0.3 34 " " 9 " " 0.6 35 " " 8 " " 0.8 36 " " 6.6 "
" 1.2 37 " " 12 " 3000 0.3 38 " " 9 " " " 39 " " 8 " " " 40 " " 6.6
" " 0.4 41 " " 12 " 6000 0.3 42 " " 9 " " " 43 " " 8 " " " 44 " "
6.6 " " " ______________________________________
Next for the sake of comparison, some comparison examples will be
described.
COMPARISON EXAMPLE 1
The pickup device is substantially similar to that shown in FIG. 1,
but no viscous liquid is used. The Q factor is 32.
COMPARISON EXAMPLE 2
The pickup device is substantially similar to that as shown in FIG.
1, but the silicon oil (the product of Shinetsu Kagaku Kogyo K.K.
sold under the trade mark KF-96) is used. The Q factor is 7.
COMPARISON EXAMPLE 3
The pickup device is substantially similar to that as shown in FIG.
1, but the silicon oil (the product of Shinetsu Kagaku Kogyo K.K.
sold under the trademark of KF-96H) is used. Because of the high
viscosity, the small diameter portion of the case of the first
embodiment cannot be completely filled.
COMPARISON EXAMPLE 4
The pickup device is substantially similar to Example 25, but no
viscous liquid is used. The Q factor is 30.
COMPARISON EXAMPLE 5
The pickup device is substantially similar in construction to
Example 25 and a silicon oil with the viscosity of 10 cs (same as
Example 2) is used. The Q factor is 5.
COMPARISON EXAMPLE 6
The pickup device is substantially similar in construction to
Example 25 and a silicon oil (same as Example 3) with the viscosity
of 12,500 cs is used. However, because of its high viscosity, the
cap 34 of the case of the second embodiment cannot be completely
filled.
Examples 1 through 24 and the Comparison Examples 1 and 2 were
inserted into an ear while Examples 25 through 40 and Comparison
Examples 4 and 5 were brought into contact with the nasal bone. All
pickup devices were connected to transceivers and mutual
communication was established when the noise level was 100 phons
(A) in order to investigate articulation or intelligibility. The
results are shown below.
______________________________________ Example Evalua- Example
Evalua- Evalua- No. tion No. tion tion
______________________________________ Example No. 1 G 17 G 34 G 2
G 18 G 35 G 3 G 19 G 36 G 4 E 20 G 37 G 5 G 21 G 38 G 6 G 22 G 39 G
7 E 23 G 40 G 8 E 24 G 41 G 9 G 25 G 42 G 10 G 26 G 43 G 11 E 27 E
44 G 12 E 28 G Comp. 13 G 29 G Example 14 G 30 G 1 P 15 G 31 E 2 P
16 G 32 E 3 P 33 G 4 P ______________________________________ Key:
E = Excellent communication G = Good communication P = Poor
communication
As described above, according to the present invention, the audio
signal can be distinctly distinguished from noise even when the
noise level is high.
* * * * *